Asymmetric multi-channel adaptive equalizer

ABSTRACT

An apparatus is disclosed to compensate for non-linear effects resulting from the transmitter, the receiver, and/or the communication channel in a communication system. A receiver of the communication system contains an image cancellation module that compensates for images generated during the modulation and/or demodulation process. The image cancellation module includes a fine carrier correction loop to correct for frequency offsets between the transmitter and receiver. The image cancellation module includes a coarse acquisition mode and a decision directed mode. The decision directed mode allows for a larger signal-to-noise ratio for the receiver when compared against the coarse acquisition mode.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/439,197, filed Apr. 4, 2012, which is a continuation of U.S.patent application Ser. No. 13/020,503, filed Feb. 3, 2011, now U.S.Pat. No. 8,160,127, which is a continuation of U.S. patent applicationSer. No. 11/878,224, filed Jul. 23, 2007, now U.S. Pat. No. 7,885,323,which claims the benefit of U.S. Provisional Patent Application No.60/898,993, filed Feb. 2, 2007, each of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to adaptive equalizers andspecifically to using an image cancellation circuit to cancel imagesgenerated during the modulation and/or demodulation process.

BACKGROUND

A digital communication system typically involves transmitting amodulated data stream from a transmitter to a receiver over acommunication channel. The communication channel can include a microwaveradio link, a satellite channel, a fiber optic cable, or a copper cableto provide some examples. A communication channel contains a propagationmedium that the modulated data stream passes through before reception bythe receiver.

The propagation medium of the communication channel introducesdistortion into the transmitted modulated data stream causing a receivedmodulated data stream to differ from the transmitted modulated datastream. Noise, signal strength variations known as fading, phase shiftvariations, or multiple path delays known as multi-path propagation canintroduce distortion into the transmitted modulated data stream. Forexample, transmission over a multiplicity of paths of different andvariable lengths, or rapidly varying delays in the propagation mediumfrom the transmitter to the receiver, may cause a change in theamplitude and/or phase of the transmitted modulated data stream.

Digital communication systems use an adjustable filter in the form of anequalizer to reduce the effect of the distortion caused by thecommunication channel. A receiver may directly set equalization filtercoefficients for known or measured communication channels. However, inmost situations the characteristics of the communication channel are notknown in advance and therefore require the use of an adaptive equalizer.Adaptive equalizers derive adjustable filter coefficients from areceived demodulated data stream.

A symmetric or a complex adaptive equalizer is an equalizer whereby thereceived demodulated data stream may be represented as complex samples.Each complex sample includes a real component and, an imaginarycomponent. The output of the complex adaptive equalizer is also complexwith a real component and an imaginary component. The imaginarycomponent and the real component of the equalized output are determinedby combining a multiplication between a delayed version of the receiveddemodulated data stream and adjustable filter coefficients of theequalizer. This complex multiplication requires two complexmultiplications and one real addition for each component of theequalized output for a total of four real multiplications and two realadditions.

One way of understanding the complex multiplication in, conventionalequalization techniques is to express it as a constrained two by tworeal matrix multiplication. The corresponding matrix used to perform themultiplication is a two by two matrix containing four real numbers. In asymmetric complex equalizer, this matrix may be constrained such thatthe diagonal elements are equal and the off-diagonal elements are thenegatives of each other to provide an example.

However, an asymmetric equalizer may relax the constraints of the two bytwo matrix by replacing the complex multiplication may with a generaltwo by two real matrix multiplication. The asymmetric equalizerimplements the two by two matrix using any four real numbers. Thesymmetric equalizer may reduce the effect of the distortion caused bythe communication channel so long as the characteristics of thecommunication system are linear. However, in practice, some effects ofthe communication channel as well as distortion caused by thetransmitter and/or receiver are not linear. For example, in amulti-channel communication signal, residual signals resulting from afrequency-inverted duplicate or mirror image of a corresponding signalof interest or one or more neighboring information channels within themulti-channel information signal are some examples of non-linear effectsrequiring the use of an asymmetric equalizer.

Therefore, what is needed is an adaptive equalizer that is capable ofcompensating for the non-linear effects resulting from the transmitter,the receiver, and/or the communication channel in a communicationsystem.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 is an illustration of a multi-channel communication systemaccording to an exemplary embodiment of the present invention.

FIG. 2A is an illustration of a multi-channel transmitter according toan exemplary embodiment of the present invention.

FIG. 2B is an illustration of a multi-channel information channelaccording to an exemplary embodiment of the present invention.

FIG. 2C is an illustration of a modulated multi-channel informationsignal according to an exemplary embodiment of the present invention.

FIG. 2D is an illustration of an upconverted multi-channel informationchannel according to another exemplary embodiment of the presentinvention.

FIG. 3A is an illustration of a multi-channel receiver according to anexemplary embodiment of the present invention.

FIG. 3B is an illustration of a received multi-channel informationchannel according to an exemplary embodiment of the present invention.

FIG. 3C is an illustration of a received multi-channel informationchannel according to another exemplary embodiment of the presentinvention.

FIG. 3D is an illustration of a multi-channel information channelaccording to an exemplary embodiment of the present invention.

FIG. 4 is an illustration of a direct conversion tuner according to anexemplary embodiment of the present invention.

FIG. 5 is an illustration of a direct current bias and imbalancecorrection module according to an exemplary embodiment of the presentinvention.

FIG. 6 is an illustration of a digital front end according to anexemplary embodiment of the present invention.

FIG. 7 is an illustration of a digital front end according to anotherexemplary embodiment of the present invention.

FIG. 8A is an illustration of an image generator according to anexemplary embodiment of the present invention.

FIG. 8B is an illustration of an image generator according to anexemplary embodiment of the present invention.

FIG. 9A is an illustration of an adaptive image canceler according to anexemplary embodiment of the present invention.

FIG. 9B is an illustration of an adaptive image canceler according toanother exemplary embodiment of the present invention.

FIG. 10 is an illustration of an adaptive image canceler according to afurther exemplary embodiment of the present invention.

FIG. 11 is an illustration of an adaptive image canceler according toanother exemplary embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers generallyindicate identical, functionally similar, and/or structurally similarelements. The drawing in which an element first appears is indicated bythe leftmost digit(s) in the reference number.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the detailed description is not meant tolimit the invention. Rather, the scope of the invention is defined bythe appended claims.

FIG. 1 is an illustration of a multi-channel communication systemaccording to an exemplary embodiment of the present invention. Amulti-channel communication system 100 includes a multi-channeltransmitter 102 to transmit a modulated representation of amulti-channel information signal 152 to a multi-channel receiver 106 viaa communication channel 104. The information signal 152 includes ncommunication channels denoted as information channels 150.1 through150.n. In an exemplary embodiment, the information channels 150.1through 150.n each have an approximate channel bandwidth of 6 or 8Megahertz (MHz) such as on a downstream cable television and data systemto provide an example. However, those skilled in the art(s) willrecognize that other bandwidths and frequency spacing may be usedwithout departing from the spirit and scope of the invention. Theinformation channel channels 150.1 through 150.n may includeinformation-bearing signals such as a Quadrature Phase-Shift Keyed(QPSK), a Phase-Shift Keyed (PSK), a Quadrature Amplitude Modulated(QAM), or a Trellis Coded Modulated (TCM) modulated signal, an analoginformation signal, an analog modulated signal, or any combinationthereof to provide some examples. Those skilled in the art(s) willrecognize that other modulation schemes may be used without departingfrom the spirit and scope of the invention. In addition, those skilledin the art(s) will also recognize that the information channel 150 neednot include an information-bearing signal without departing from thespirit and scope of the invention.

As shown in FIG. 1, the multi-channel transmitter 102 produces atransmitted multi-channel information signal 156 by modulating themulti-channel information signal 152. The transmitted multi-channelinformation signal 156 passes through the communication channel 104 toproduce a received multi-channel information signal 158. Thecommunication channel 104 may include a microwave radio link, asatellite channel, a fiber optic cable, a hybrid fiber optic cablesystem, or a copper cable to provide some examples. The communicationchannel 104 contains a propagation medium, that the transmittedmulti-channel information signal 156 passes through before reception bythe multi-channel receiver 106. The propagation medium of thecommunication channel 104 introduces distortion into the transmittedmulti-channel information signal 156 to produce the receivedmulti-channel information signal 158. Noise such as, but not limited to,thermal noise, burst noise, impulse noise, interference, signal strengthvariations known as fading, phase shift variations, or multiple pathdelays as multi-path propagation to provide some examples can introducedistortion into the transmitted multi-channel information signal 156.

Referring back to FIG. 1 the multi-channel communication system 100includes the multi-channel receiver 106 to receive the channelinformation signal 158. The multi-channel receiver 106 produces amulti-channel information signal 154 by demodulating the receivedmulti-channel information signal 158 then separating a demodulatedrepresentation of the received multi-channel information signal 158 inton communication channels. The multi-channel information signal 154includes n communication channels denoted as information channels 160.1through 160.n. Then information channels 160.1 through 160.n may includeinformation-bearing signals such as a Quadrature Phase-Shift Keyed(QPSK), a Phase-Shift Keyed (PSK), a Quadrature Amplitude Modulated(QAM), or a Trellis Coded Modulated (TCM) modulated signal, an analoginformation signal, an analog modulated signal, or any combinationthereof, to provide some examples. Those skilled in the art(s) willrecognize that other modulation schemes may be used without departingfrom the spirit and scope of the invention. In addition, those skilledin the art(s) will also recognize that the n image corrected basebandinformation channels 160.1 through 160.n need not include aninformation-bearing signal without departing from the spirit and scopeof the invention.

FIG. 2A is an illustration of a multi-channel transmitter according toan exemplary embodiment of the present invention. In this exemplaryembodiment, the multi-transmitter 102 modulates the multi-channelinformation signal 152 to produce the transmitted multi-channelinformation signal 156. Those skilled in the art(s) will recognize thatthe multi-channel transmitter 102 may be implemented according to anysuitable modulation technique without departing from the spirit andscope of the invention.

The multi-channel transmitter 102 includes a mixer 202, a summer 204, amixer 206, and a complex operation module 208. The mixer 202 produces acorresponding single channel upconverted information signal 250.1through 250.n by converting the information signal 152 to anintermediate frequency (IF). More specifically, the mixer 202 containsn<mixers 202.1 through 202.n to upconvert an information-bearing signal,if present, within a corresponding information channel 150.1 through150.n to a corresponding carrier frequency ω₁ through ω_(n), denoted ase^(jω) ¹ ^(t) through e^(jω) ^(n) ^(t) in FIG. 2A. For example, themixer 202.1 upconverts the information-bearing signal, if present,within the information channel 150.1 to a carrier frequency of ω₁.

The summer 204 combines the single channel upconverted informationsignals 250.1 through 250.n to produce a multi-channel informationsignal 254. The mixer 206 produces an upconverted multi-channelinformation signal 256 by upconverting the multi-channel informationsignal 254 to a corresponding carrier frequency ω_(c), denoted e^(jω)^(c) ^(t) in FIG. 2A. In an exemplary embodiment, the carrier frequencyω_(c) may range from approximately 108 MHz to approximately 860 MHz.Those skilled in the art(s) will recognize that other carrierfrequencies may be used without departing from the spirit and scope ofthe invention. The upconverted multi-channel information signal 256 maybe expressed in a complex form including a real component and animaginary component. The complex operation module 208 operates on theupconverted multi-channel information signal 256 by isolating the realcomponent of the upconverted multi-channel information signal 256 toproduce the transmitted multi-channel information signal 156.

FIG. 2B is an illustration of a multi-channel information signalaccording to an exemplary embodiment of the present invention. As shownin FIG. 2B, each information channel 150.1 through 150.n in themulti-channel information signal 152 may include a corresponding signalof interest 202.1 through 202.n corresponding to an embedded informationsignal. Each information channel 150.1 through 150.n may additionallyinclude a residual signal 200.1 through 200.n , resulting from, but notlimited to, a frequency-inverted duplicate or mirror image of acorresponding signal of interest 202.1 through 202.n, one or moreneighboring information channels within the multi-channel informationsignal 152, or any other suitable source to provide some examples. Thespectral representation of the multi-channel information signal 152 asshown in FIG. 2B is for illustrative purposes only. Those skilled in theart(s) will recognize the multi-channel information signal 152 mayinclude any suitable spectral representation without departing from thespirit and scope of the invention. Referring back to FIG. 2B, thesignals of interest 202.1 through 202.n occupy the frequency range from0 Hz to W Hz. Likewise, the residual signals 200.1 through 200.n occupythe frequency range from −W Hz to 0 Hz. For example, the signals ofinterest 202.1 through 202.n may occupy the frequency spectrum fromapproximately 0 Hz to approximately 3 MHz. For this scenario, theresidual signals 200.1 through 200.n may occupy the frequency spectrumfrom approximately −3 MHz to approximately 0 Hz.

The multi-channel transmitter 102 uses a process to filter or cancel theresidual signals 200.1 through 200.n throughout the modulation process.However, if the filtering or canceling of the residual signals 200.1through 200.n within the multi-channel information signal 152 during themodulation process is not complete, an attenuated version of theresidual, signals 200.1 through 200.n remains in the transmittedmulti-channel information signal 156. In addition, frequency offsetspresent in the multi-channel transmitter 102, such as an offset betweenthe in-phase components and the quadrature components of themulti-channel information signal 152 to provide an example, may alsocause an attenuated version of the residual signals 200.1 through 200.nto remain in the transmitted multi-channel information signal 156.

FIG. 2C is an illustration of a transmitted multi-channel informationsignal 156 according to an exemplary embodiment of the presentinvention. FIG. 2C exemplifies a scenario where the multi-channeltransmitter 102 does not substantially cancel the image of acorresponding signal of interest 202.1 through 202.n within acorresponding residual signal 200.1 through 200.n. In addition, afrequency offset, such an offset in frequency between the in-phasecomponents and the quadrature components of the corresponding signal ofinterest 202.1 through 202.n, causes one or more neighboring informationchannels within the multi-channel information signal 152 to be impressedwithin the corresponding residual signal 200.1 through 200.n. Thisexemplary embodiment demonstrates the transmitted multi-channelinformation signal 156 including having an odd number of channels, suchthe information channels 150.1 through 150.5 to provide an example.

As shown in FIG. 2C, a left most channel of the transmittedmulti-channel information signal 156 occupying the frequency bandwidthfrom −n*W+ω_(c) to −(n−2)*W+ω_(c) contains a modulated version of thesignal of interest 202.1, where n represents the number of communicationchannels in the transmitted multi-channel information signal 156, ω_(c)represents the carrier frequency used to transmit the transmittedmulti-channel information signal 156, and 2 W represents the bandwidthof a corresponding signal of interest 202.1 through 202.n includedwithin the multi channel information signal 152. The left most channelof the transmitted multi-channel information signal 156 may also includean attenuated version of the residual signal 200.1. The residual signal200.1 may include, but is not limited to, a mirror image of the signalof interest 202.n, other communication signals from one or moreneighboring information channels within the multi-channel informationsignal 152, or any other communication signal to provide some examples.

A next modulated channel occupying the frequency bandwidth from−(n−2)*W+ω_(c) to −(n−4)*W+ω_(c) contains a modulated version of thesignal of interest 202.2. The next modulated channel of the transmittedmulti-channel information signal 156 may also include an attenuatedversion of the residual signal 200.2. The residual signal 200.2 mayinclude, but is not limited to, a mirror image of the signal of interest202.(n−1), other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

A median modulated channel occupying the frequency bandwidth from−W+ω_(c) to W+ω_(c) contains a modulated version of the signal ofinterest 202.((n+1)/2). The median modulated channel of the transmittedmulti-channel information signal 156 may also include an attenuatedversion of the residual signal 200.((n+1)/2). The residual signal200.((n+1)/2) may include, but is not limited to, a mirror image of thesignal of interest 202.((n+1)/2), other communication signals from oneor more neighboring information channels within the multi-channelinformation signal 152, or any other communication signal to providesome examples.

A final modulated channel occupying the frequency bandwidth from−2)*W+ω_(c) to n*W+ω_(c) contains a modulated version of the signal ofinterest 202.n. The final modulated channel of the transmittedmulti-channel information signal 156 may also include an attenuatedversion of the residual signal 200.n. The residual signal 200.n mayinclude, but is not limited to, a mirror image of the signal of interest202.1, other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

FIG. 2D is an illustration of a transmitted multi-channel informationsignal 156 according to another exemplary embodiment of the presentinvention. FIG. 2D exemplifies a scenario where the multi-channeltransmitter 102 does not substantially cancel the image of acorresponding signal of interest 202.1 through 202.n within acorresponding residual signal 200.1 through 200.n. In addition, afrequency offset, such an offset in frequency between the in-phasecomponents and the quadrature components of the corresponding signal ofinterest 202.1 through 202.n, causes one or more neighboring informationchannels within the multi-channel information signal 152 to be impressedwithin the corresponding residual signal 200.1 through 200.n. Thisexemplary embodiment demonstrates the transmitted multi-channelinformation signal 156 including having an even number of channels, suchthe information channels 150.1 through 150.4 to provide an example.

As shown in FIG. 2D, a left most channel of the transmittedmulti-channel information signal 156 occupying the frequency bandwidthfrom −n*W+ω_(c) to −(n−2)*W+ω_(c) contains a modulated version of thesignal of interest 202.1, where n represents the number of communicationchannels in the transmitted multi-channel information signal 156, ω_(c)represents the carrier frequency used to transmit the transmittedmulti-channel information signal 156, and W represents the bandwidth ofa corresponding signal of interest 202.1 through 202.n included withinthe multi-channel information signal 152. The left most channel of thetransmitted multi-channel information signal 156 may also include anattenuated version of the residual signal 200.1. The residual signal200.1 may include, but is not limited to, a mirror image of the signalof interest 202.n, other communication signals from one or moreneighboring information channels within the multi-channel informationsignal 152, or any other communication signal to provide some examples.

A next modulated channel occupying the frequency bandwidth from−(n−2)*W+ω_(c) to −(n−4)*W+ω_(c) contains a modulated version of thesignal of interest 202.2. The next modulated channel of the transmittedmulti-channel information signal 156 may also include an attenuatedversion of the residual signal 200.2. The residual signal 200.2 mayinclude, but is not limited to, a mirror image of the signal of interest202.(n−1), other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

The corresponding carrier frequency ω_(c) lies adjacent to a firstmedian modulated channel occupying the frequency bandwidth from −2W+ω_(c) to ω_(c) and a second median modulated channel occupying thefrequency bandwidth from ω_(c) to 2 W+ω_(c). The first median modulatedchannel contains a modulated version of the signal of interest202.(n/2). The first median modulated channel of the transmittedmulti-channel information signal 156 may also include an attenuatedversion of the residual signal 200.(n/2). The residual signal 200.2 mayinclude, but is not limited to, a minor image of the signal of interest202.((n/2)+1), other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples. The secondmedian modulated channel contains a modulated version of the signal ofinterest 202.((n/2)+1). The second median modulated channel of thetransmitted multi-channel information signal 156 may also include anattenuated version of the residual signal 200.((n/2)+1). The residualsignal 200.((n/2)+1) may include, but is not limited to, a mirror imageof the signal of interest 202.(n/2), other communication signals fromone or more neighboring information channels within the multi-channelinformation signal 152, or any other communication signal to providesome examples.

A final modulated channel occupying the frequency bandwidth from(n−2)*W+ω_(c) to n*W+ω_(c) contains a modulated version of the signal ofinterest 202.n. The final modulated channel of the transmittedmulti-channel information signal 156 may also include an attenuatedversion of the residual signal 200.n. The residual signal 200.n mayinclude, but is not limited to, a mirror image of the signal of interest202.1, other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

FIG. 3A is an illustration of a multi-channel receiver according to anexemplary embodiment of the present invention. In this exemplaryembodiment, the multi-channel receiver 106 receives the transmittedmulti-channel information signal 156 from the communication channel 104.The multi-channel receiver 106 produces a multi-channel informationsignal 154 by demodulating the received multi-channel information signal158 then separating a demodulated representation of the receivedmulti-channel information signal 158 into n communication channels. Themulti-channel information signal 154 includes n communication channelsdenoted as information channels 160.1 through 152.n. The multi-channelreceiver 106 includes a direct conversion toner 302, a direct current(DC) bias and imbalance correction module 304, a digital front end 306,and an image canceler 308. The multi-channel receiver 106 will beexplained in further detail referring to FIG. 4 through FIG. 10.

FIG. 3B is an illustration of a received multi-channel informationsignal 158 according to an exemplary embodiment of the presentinvention. FIG. 3B exemplifies a scenario where the multi-channeltransmitter 102 does not substantially cancel the image of acorresponding signal of interest 302.1 through 302.n within acorresponding residual signal 300.1 through 300.n. In addition, afrequency offset, such an offset in frequency between the in-phasecomponents and the quadrature components of the corresponding signal ofinterest 302.1 through 302.n, causes one or more neighboring informationchannels within the multi-channel information signal 152 to be impressedwithin the corresponding residual signal 300.1 through 300.n. Further,the communication channel may introduce noise such as, but not limitedto, thermal noise, burst noise, impulse noise, interference, signalstrength variations known as fading, phase shift variations, or multiplepath delays known as multi-path propagation to provide some into thereceived multi-channel information signal 158. This exemplary embodimentdemonstrates the received multi-channel information signal 158 includinghaving an odd number of channels, such the information channels 150.1through 150.5 to provide an example.

As shown in FIG. 3B, a left most channel of the received multi-channelinformation signal 158 occupying the frequency bandwidth from −n*W+ω_(c)to −(n−2)*W+ω_(c) contains a modulated version of the signal of interest302.1, where n represents the number of communication channels in thereceived multi-channel information signal 158, ω_(c) represents thecarrier frequency used to transmit the received multi-channelinformation signal 158, and 2 W represents the bandwidth of acorresponding signal of, interest 302.1 through 302.n included withinthe multi-channel information signal 152. The left most channel of thereceived multi-channel information signal 158 may also include anattenuated version of the residual signal 300.1. The residual signal300.1 may include, but is not limited to, a mirror image of the signalof interest 302.n, other communication signals from one or moreneighboring information channels within the multi-channel informationsignal 152, or any other communication signal to provide some examples.

A next modulated channel occupying the frequency bandwidth from−(n−2)*W+ω_(c) to −(n−4)*W+ω_(c) contains a modulated version of thesignal of interest 302.2. The next modulated channel of the receivedmulti-channel information signal 158 may also include an attenuatedversion of the residual signal 300.2. The residual signal 300.2 mayinclude, but is not limited to, a mirror image of the signal of interest302.(n−1), other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

A median modulated channel occupying the frequency bandwidth from−W+ω_(c) to W+ω_(c) contains a modulated version of the signal ofinterest 302.((n+1)/2). The median modulated channel of the receivedmulti-channel information signal 158 may also include an attenuatedversion of the residual signal 300.((n+1)/2). The residual signal300.((n+1)/2) may include, but is not limited td, a mirror image of thesignal of interest 302.((n+1)/2), other communication signals from oneor more neighboring information channels within the multi-channelinformation signal 152, or any other communication signal to providesome examples.

A final modulated channel occupying the frequency bandwidth from(n−2)*W+ω_(c) to n*W+ω_(c) contains a modulated version of the signal ofinterest 302.n. The final modulated channel of the receivedmulti-channel information signal 158 may also include an attenuatedversion of the residual signal 300.n. The residual signal 300.n mayinclude, but is not limited to, a mirror image of the signal of interest302.1, other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

FIG. 3C is an illustration of a received multi-channel informationsignal 158 according to another exemplary embodiment of the presentinvention. FIG. 3C exemplifies a scenario where the multi-channeltransmitter 102 does not substantially cancel the image of acorresponding signal of interest 302.1 through 302.n within acorresponding residual signal 300.1 through 300.n. In addition, afrequency offset, such an offset in frequency between the in-phasecomponents and the quadrature components of the corresponding signal ofinterest 302.1 through 302.n, causes one or more neighboring informationchannels within the multi-channel information signal 152 to be impressedwithin the corresponding residual signal 300.1 through 300.n. Thisexemplary embodiment demonstrates the received multi-channel informationsignal 158 including having an even number of channels, such theinformation channels 150.1 through 150.4 to provide an example. Further,the communication channel may introduce noise such as, but not limitedto, thermal noise, burst noise, impulse noise, interference, signalstrength variations known as fading, phase shift variations, or multiplepath delays known as multi-path propagation to provide some into thereceived multi-channel information signal 158.

As shown in FIG. 3C, a left most channel of the received multi-channelinformation signal 158 occupying the frequency bandwidth from −n*W+ω_(c)to −(n−2)*W+ω_(c) contains a modulated version of the signal of interest302.1, where n represents the number of communication channels in thereceived multi-channel information signal 158, ω_(c) represents thecarrier frequency used to transmit the received multi-channelinformation signal 158, and W represents the bandwidth of acorresponding signal of interest 302.1 through 302.n included within themulti-channel information signal 152. The left most channel of thereceived multi-channel information signal 158 nay also include anattenuated version of the residual signal 300.1. The residual signal300.1 may include, but is not limited to, a mirror image of the signalof interest 302.n, other communication signals from one or moreneighboring information channels within the multi-channel informationsignal 152, or any other communication signal to provide some examples.

A next modulated channel occupying the frequency bandwidth from−(n−2)*W+ω_(c) to −(n−4)*W+ω_(c) contains a modulated version of thesignal of interest 302.2. The next modulated channel of the receivedmulti-channel information signal 158 may also include an attenuatedversion of the residual signal 300.2. The residual signal 300.2 mayinclude, but is not to, a minor image of the signal of interest302.(n−1), other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

The corresponding carrier frequency ω_(c) lies adjacent to a firstmedian modulated channel occupying the frequency bandwidth from −2W+ω_(c) to ω_(c) and a second median modulated channel occupying thefrequency bandwidth from ω_(c) to 2 W+ω_(c). The first median modulatedchannel contains a modulated version of the signal of interest302.(n/2). The first median modulated channel of the receivedmulti-channel information signal 158 may also include an attenuatedversion of the residual signal 300.(n/2). The residual signal 300.2 mayinclude, but is not limited to, a mirror image of the signal of interest302.((n/2)+1), other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples. The secondmedian modulated channel contains a modulated version of the signal ofinterest 302.((n/2)+1). The second median modulated channel of thereceived multi-channel information signal 158 may also include anattenuated version of the residual signal 300.((n/2)+1). The residualsignal 300.((n/2)+1) may include, but is not limited to, a mirror imageof the signal of interest 302.(n/2), other communication signals fromone or more neighboring information channels within the multi-channelinformation signal 152, or any other communication signal to providesome examples.

A final modulated channel occupying the frequency bandwidth from(n−2)*W+ω_(c) to n*W+ω_(c) contains a modulated version of the signal ofinterest 302.n. The final modulated channel of the receivedmulti-channel information signal 158 may also include an attenuatedversion of the residual signal 300.n. The residual signal 300.n mayinclude, but, is not limited to, a mirror image of the signal ofinterest 302.1, other communication signals from one or more neighboringinformation channels within the multi-channel information signal 152, orany other communication signal to provide some examples.

FIG. 3D is an illustration of a received multi-channel informationsignal 158 according to an exemplary embodiment of the presentinvention. As shown in FIG. 31), each information channel 150.1 through152.n in the multi-channel information signal 154 may include acorresponding signal of interest 302.1 through 302.n corresponding to aprocessed representation of a corresponding signal of interest 202.1through 202.n. Each information channel 150.1 through 152.n mayadditionally include a residual signal 300.1 through 300.n. Whencompared to the residual signal 200.1 through 200, the residual signal300.1 through 300.n is substantially free of the interference anddistortion caused by, but not limited to, the frequency-invertedduplicate or mirror image of a corresponding signal of interest 202.1through 202.n, one or more neighboring information channels within themulti-channel information signal 152, or any other, suitable source toprovide some examples. The spectral representation of the multi-channelinformation signal 152 as shown in FIG. 3D is for illustrative purposesonly. Those skilled in the art(s) will recognize the multi-channelinformation signal 152 may include any suitable spectral representationwithout departing from the spirit and scope of the invention.

FIG. 4 is an illustration of a direct conversion tuner according to anexemplary embodiment of the present invention. The direct conversiontuner 302 is capable of receiving the received multi-channel informationsignal 158 from the communication channel 104. The receivedmulti-channel information signal 158 may include an odd number ofchannels, as shown in FIG. 3B or an even number of channels, as shown inHG. 3C. The direct conversion tuner 302 produces a digitized informationsignal 350 by downconverting the received multi-channel informationsignal 158 directly to baseband in a single downconversion.

The direct conversion tuner 302 includes a mixer 402, a filter 404, anautomatic gain control (AGC) module 406, and an analog to digitalconverter (ADC) 408. The mixer 402 multiplies the received multi-channelinformation signal 158 by a corresponding carrier frequency todown-convert an in-phase component and a quadrature component of thereceived multi-channel information signal 158 directly to baseband. Morespecifically, the mixer 402.1 extracts the in-phase component of thereceived multi-channel information signal 158, denoted as in-phaseinformation signal 450.1, by multiplying the received multi-channelinformation signal 158 by cos((ω_(c)+ω_(t))t+φ). Ideally, the carrierfrequency ω_(c) of the direct conversion tuner 302 is substantiallyequivalent to, the carrier frequency of ω_(c) used by the multi-channeltransmitter 102. However, in practice, the carrier frequency ω_(c) ofthe direct conversion tuner 302 may be offset from the carrier frequencyof ω_(c) used by the multi-channel transmitter 102 by a phase offset ofφ and a frequency offset of ω_(t). For example, the propagation mediumor any variation in an oscillator of the direct conversion tuner 302 cancause the carrier offset φ and the frequency offset of ω_(t). Likewise,the mixer 402.2 extracts the quadrature phase component of the receivedmulti-channel information signal 158, denoted as a quadrature phaseinformation signal 450.2, by multiplying the received multi-channelinformation signal 158 by sin((ω_(c)+ω_(t))t−φ).

The filters 404 remove out of band interference resulting from, but notlimited to, noise such as, but not limited to, thermal noise, burstnoise, impulse noise, adjacent channels interference to provide someexamples to produce a corresponding filtered information signal 452. Thefilter 404.1 produces a filtered in-phase information signal 452.1 byremoving out of band signals, noise, and interference from the in-phaseinformation signal 450.1 according to a transfer function h1. Similarly,the filter 404.2 produces a filtered quadrature phase information signal452.2 by removing out of band signals, noise, and interference from thequadrature phase information signal 450.2 according to a transferfunction h2. The transfer function h1 and the transfer function h2 neednot be identical and may differ as a result of imbalances between thein-phase component and quadrature component of the receivedmulti-channel information signal 158 to provide an example. By allowingthe transfer h1 and the transfer function h2 to differ, the directconversion tuner 302 may be designed so as to compensate for a frequencyselective imbalance between the in-phase component and quadrature phasecomponent of the received multi-channel information signal 158. On theother hand, if h1 and h2 differ in an unpredictable manner, they mayintroduce an unknown frequency selective imbalance between the in-phasecomponent and quadrature phase component of the received multi-channelinformation signal 158. In an exemplary embodiment, the bandwidth of thetransfer function h1 and the bandwidth of transfer function h2 aresubstantially large enough to allow a downconverted representation ofthe multi-channel information signal 254 as shown in FIG. 2 to passthrough filter 404.1 and filter 404.2 respectively without substantialdistortion or stopband attenuation.

The AGC 406 amplifies and/or attenuates a corresponding filteredinformation signal 452 to, produce a corresponding magnitude correctedinformation signal 454. The AGC 406.1 produces a magnitude corrected inphase information signal 454.1 by amplifying and/or attenuating thefiltered in-phase information signal 452.1. Likewise, the AGC 406.2produces a magnitude corrected quadrature phase information signal 454.2by amplifying and/or attenuating filtered quadrature phase informationsignal 452.2. The gain of AGC 406.1 and AGC 406.2 need not be identicaland can differ to correct a magnitude imbalance between the in-phasecomponent and quadrature component of the of the received multi-channelinformation signal 158. In an exemplary embodiment, AGC 406 is optional;the filtered information signal 452 may be directly used as an input toADC 408.

The ADC 408 converts a corresponding magnitude corrected informationsignal 454 from an analog signal to produce a corresponding digitizedinformation signal 350. The digitized information signal 350.1represents a digitized down-converted version of the in-phase componentof the received multi-channel information signal 158 directly tobaseband. The digitized information signal 350.2 represents a digitizeddown-converted version of the quadrature component of the receivedmulti-channel information signal 158 directly to baseband.

FIG. 5 is an illustration of a direct current bias and imbalancecorrection module according to an exemplary embodiment of the presentinvention. The direct current (DC) bias and imbalance correction module304 substantially removes a DC offset generated from the directconversion tuner 302. The DC bias and imbalance correction module 304may substantially negate an imbalance between the in-phase component andthe quadrature component present in the information-bearing signalwithin the received multi-channel information signal 158.

The DC bias and imbalance correction module 304 includes a summer 502,an AGC 504, an in-phase balance correction module 506, a quadraturebalance correction module 508, and a summer 510. The summer 502 removesthe DC offset impressed onto the digitized information signal 350 duringthe down-conversion of the received multi-channel information signal158. The summer 502.1 substantially removes or cancels the DC offsetpresent in the digitized information signal 350.1 to produce a DCadjusted information signal 550.1 by subtracting the DC offset, denotedas I DC Offset, from the digitized information signal 350.1. Likewise,the summer 502.2 removes or cancels the DC offset present in thedigitized information signal 350.2 to produce a DC adjusted informationsignal 550.2 by subtracting the DC offset, denoted as a Q DC Offset,from the digitized information signal 350.2.

The AGC 504 operates upon the DC adjusted information signal 550 tocorrect for differences in magnitude between the in-phase component andthe quadrature component of the received multi-channel informationsignal 158. The differences in magnitude may result from a difference inthe attenuation of the in-phase component and the quadrature componentof either the multi-channel transmitter 102 or the multi-channelreceiver 106 to provide some examples. The AGC 504.1 attenuates the DCadjusted information signal 550.1 to, produce a magnitude adjustedinformation signal 552.1. Similarly, the AGC 504.2 attenuates the DCadjusted information signal 550.2 to produce a magnitude adjustedinformation signal 552.2.

The phase offset of φ may be determined by the cross-correlation of thein-phase component and the quadrature component of the receivedmulti-channel information signal 158 in either direct conversion tuner302 after the mixer 402 or before the phase balance correction module506 and quadrature balance correction module 508 of the direct current(DC) bias and imbalance correction module 304. The in-phase balancecorrection module 506 operates in conjunction with the quadraturebalance correction module 508 and the summer 510 to correct for phaseoffset of φ resulting from the down-conversion of the receivedmulti-channel information signal 158 by direct conversion tuner 302.More specifically, the in-phase balance correction module 506.1attenuates the magnitude adjusted information signal 552.1 by cos(φ) toproduce a signal 554.1, while in-phase balance correction module 506.2attenuates the magnitude adjusted information signal 552.2 by cos(φ) toproduce a signal 554.2. The quadrature balance correction module 508.1attenuates the magnitude adjusted information signal 552.1 by −sin(φ) toproduce signal, a 556.1, while quadrature balance correction module508.2 attenuates the magnitude adjusted information signal 552.2 by−sin(φ) to produce signal a 556.2. The summer 510 then compensates forphase offset of φ resulting from the down-conversion of the receivedmulti-channel information signal 158 by combining corresponding signals554 and corresponding, signals 556. The offset corrected signal 352 is acomplex information signal including a phase corrected in-phasecomponent and a phase corrected quadrature component. The summer 510.1generates the phase corrected in-phase component by combining, thesignal 554.1 with the signal 556.2, while the summer 510.2 generates thephase corrected in quadrature component by combining signal the 554.2with signal the 556.1. As a result, the in-phase component of the offsetcorrected signal 352 may be represented as:

$\begin{matrix}{\frac{{I*{\cos(\phi)}} - {Q*{\sin(\phi)}}}{{\cos^{2}(\phi)} - {\sin^{2}(\phi)}},} & (1)\end{matrix}$where φ represents the phase offset of φ shown in FIG. 4, I representsthe magnitude adjusted information signal 552.1, and Q represents themagnitude adjusted information signal 552.2. Similarly, the quadraturecomponent of the offset corrected signal 352 may be represented as:

$\begin{matrix}{\frac{{{- I}*{\sin(\phi)}} + {Q*{\cos(\phi)}}}{{\cos^{2}(\phi)} - {\sin^{2}(\phi)}},} & (2)\end{matrix}$where φ represents the phase offset of φ shown in FIG. 4, I representsthe magnitude adjusted information signal 552.1, and Q represents themagnitude adjusted information signal 552.2.

FIG. 6 is an illustration of a digital front end according to anexemplary embodiment of the present invention. The digital front end 600separates the offset corrected signal 352 into the n channelscorresponding to the information channels 150.1 through 150.n as shownin FIG. 1 and FIG. 2. The digital front end 600 may represent anexemplary implementation of the digital front end 306 as shown in FIG.3. The multi-channel receiver 106 may include the digital front end 600for a received multi-channel information signal 158 including, anunknown frequency offset, denoted as of ω_(t) as shown in FIG. 4. Theunknown frequency offset of ω_(t) is capable of being accuratelymeasured and completely compensated for in by the multi-channel receiver106, and/or the unknown frequency offset of ω_(t) is substantially zero.

The digital front end 600 includes a mixer 602, a half-band filter 604,a decimator 606, and a Nyquist filter 608. The mixer 602 furtherdown-converts the offset corrected signal 352 to allow for theseparation into the n information channels of the received multi-channelinformation signal 158. The mixer 602 comprises of n multipliers 602.1through 602.n to down convert the offset corrected signal 352 by acorresponding downconversion frequency ω_(m1) through ω_(mN), denoted ase^(jω) ^(m1) ^(t) through e^(jω) ^(mN) ^(t) FIG. 6. The frequency ofindividual downconversion frequencies ω_(m1) through ω_(mN) is chosen toallow for the downconversion of a corresponding information channel tobaseband. For the received multi-channel information signal 158 havingan odd number of information channels, the carrier frequency ω_(c) ofthe multi-channel transmitter 102 and/or the multi-channel receiver 106may be selected to allow for a downconversion frequency, namelyω_(m(N/2)), corresponding to the median information channel, such as themedian modulated channel in FIG. 2C to be substantially zero. Forexample, for a multi-channel receiver containing five informationchannels, the carrier frequency ω_(c) of the multi-channel transmitter102 and/or the multi-channel receiver 106 is selected to such thatdownconversion frequency corresponding to the middle informationchannel, namely ω_(m3), is substantially zero. This allows the frequencyof the downconversion frequency ω_(m2) to be a negative representationof the downconversion frequency ω_(m4). Likewise, the frequency of thedownconversion frequency ω_(m1) is a negative representation of thedownconversion frequency ω_(m5).

The half-band filter 604 operates in conjunction with the decimator 606to eliminate out of channel interference from a corresponding channel.The half-band filter 604 comprises n half band filters 604.1 through604.n to filter a corresponding baseband information channel 650.1through 650.n. In an exemplary embodiment, the half-band filter 604 hasa bandwidth substantially greater than the bandwidth of thecorresponding the information channels 150.1 through 150.n. Thedecimator 606 comprises n decimators 606.1 through 606.n to resample acorresponding filtered information signal 652.1 through 652.n. Thereduction in the bandwidth of the baseband information channel 650.1through 650.n by the corresponding half band filter 604.1 through 604.nallows for a reduction in the sample rate of the digital front end 306by a corresponding decimator 606.1 through 606.n.

The Nyquist filter 608 produces a single channel information signal358.1 through 358.n by filtering a corresponding decimated basebandinformation signal 654. More specifically, the Nyquist filter comprisesn Nyquist filters 608.1 through 608.n to filter a correspondingdecimated baseband information signal 654.1 through 654.n. The Nyquist608 filters the decimated baseband information signal 654 with a filterwhose shape matches the pulse shape of the information channel 150. Inaddition to limiting the amount of noise spectrum passed onto subsequentstages, the Nyquist 608 correlates the decimated baseband informationsignal 654 with the transmitted communication signal.

FIG. 7 is an illustration of a digital front end according to anotherexemplary embodiment of the present invention. As with the digital frontend 600, the digital front end 700 separates the offset corrected signal352 into the n channels corresponding to the information channels 150.1through 150.n as shown in FIG. 1 and FIG. 2. The multi-channel receiver106 may include the digital front end 600 for a received multi-channelinformation signal 158 including an unknown frequency offset, denoted asof ω_(t) as shown in FIG. 4. The unknown frequency offset of ω_(t) iscapable of being accurately measured and completely compensated for inby the multi-channel receiver 106, and/or the unknown frequency offsetof ω_(t) is substantially zero.

The digital front end 700 includes a mixer 602, a half-band filter 604,a decimator 606, a mixer 704, a variable interpolator decimator (VID)708, an automatic gain control (AGC) module 712, and a Nyquist filter708. The mixer 602, the half-band filter 604, and the decimator 606operate in a substantially similar manner as described in FIG. 6. Themixer 704 further down-converts the decimated baseband informationsignal 654 to baseband to remove residual frequency offsets such as theunknown frequency offset of ω_(t) to provide an example. Morespecifically, mixer 704 comprises of n mixers 704.1 through 704.n todown-convert a corresponding decimated baseband information signal 654.1through 654.n using a carrier frequency ω_(x1) through ω_(xN), denotedas e^(jω) ^(x1) ^(t) through e^(jω) ^(xN) ^(t).

The VID 708 comprises of n VIDs 708.1 through 708.n to resample acorresponding information signal 752 from the sampling rate used by A/D408 as shown in FIG. 4. In an exemplary embodiment, the VID 708resamples the to an integer number of samples per symbol as inaccordance with the modulation scheme of the received multi-channelinformation signal 158. The AGC module 712 comprises of n AGC modules708.1 through 708.n to amplify and/or attenuate a correspondingresampled information signal 756.1 through 756.n. AGC 712 ensures theresampled information signals 756.1 through 756.n have substantiallyequal amplitudes.

The Nyquist filter 708 produces a single channel information signal354.1 through 354.n by filtering a corresponding amplitude correctedinformation signal 760.1. More specifically, the Nyquist filtercomprises n Nyquist filters 708.1 through 708.n to filter acorresponding amplitude corrected information signal 760. The Nyquist708 filters the resampled information signal 756 with a filter whoseshape matches the pulse shape of the information channel 150. Inaddition to limiting the amount of noise spectrum passed onto subsequentstages, the Nyquist 708 provides the multi-channel receiver 106 with astronger signal to work with by correlating the resampled informationsignal 756 with the pulse shape of the transmitted communication signalover the symbol period, according to the well-known principle of matchedfiltering.

FIG. 8A is an illustration of an image reference generator according toan exemplary embodiment of the present invention. An image referencegenerator 800 may be used by the multi-channel receiver 106 to produce acorresponding reference residual signal 356.1 through 356.n in FIG. 3A.The image reference generator 800 operates upon a correspondinginformation channel 160.1 through 160.n to produce the correspondingreference residual signal 356.1 through 356.n to be used by the adaptivedecision feedback equalizer (DFE), image canceler module 312. The DFE,image canceler module 312, shown in FIG. 9 and FIG. 10, utilizes thereference residual signal 356 to suppress or remove the referenceresidual signal 356.1 through 356.n from a corresponding informationchannel in the received multi-channel information signal 158. In anexemplary embodiment, the image reference generator 801 is used inconjunction with digital front end 700 where the frequency offset ofω_(t) as shown in FIG. 4 may be substantially zero or accuratelymeasured and completely compensated for by the multi-channel receiver106.

The image reference generator 800 includes an AGC module 814. Withoutthe AGC module 814 or other means to control the adaptation step size,the least-squares algorithm used by the image canceler 308 may becomeunstable for large image signals. The AGC module 814 comprises n AGCmodules 814.1 through 814.n to amplify and/or attenuate the magnitude ofa corresponding information channel 160.1 through 160.n to produce thecorresponding, reference residual signal 356.1 through 356.n. Forexample, the AGC module 814.1 amplifies and/or attenuates the magnitudeof the information channel 160.n to produce the reference residualsignal 356.1.

FIG. 8B is an illustration of an image reference generator according toanother exemplary embodiment of the present invention. The imagereference generator 801 may be used by multi-channel receiver 106 toproduce a corresponding reference residual signal 356.1 through 356.n inFIG. 3A. The image reference generator 801 operates upon a correspondinginformation channel 160.1 through 152.n to produce the correspondingreference residual signal 356.1 through 356.n to be used by the adaptivedecision feedback equalizer (DFE), image canceler module 312. The DFE,image canceler module 312, shown in FIG. 9 and FIG. 10, utilizes thereference residual signal 356 to suppress or remove the referenceresidual signal 356.1 through 356.n from a corresponding informationchannel in the received multi-channel information signal 158. In anexemplary embodiment, the image reference generator 801 is used inconjunction with digital front end 700 where the frequency offset ofω_(t) as shown in FIG. 4 may not be substantially zero nor accuratelymeasured and completely compensated for.

The image reference generator 801 includes a conjugate module 802, amixer 804, a variable interpolator decimator (VID) 808, an automaticgain control (AGC) module 810, and a Nyquist filter 812. The conjugatemodule 802 conjugates a corresponding decimated baseband informationsignal 654. More specifically, conjugate module 802 comprises nconjugate modules 802.1 through 801.n to conjugate a correspondingdecimated baseband information signal 654.n through 654.1 to produce aconjugated baseband image 850.1 through 850.n. In other words, theconjugate module 802 frequency inverts or mirrors a decimated version ofa corresponding information channel 150.1 through 150.n of the receivedmulti-channel information signal 158 as shown in FIG. 3B. For example,the conjugate module 802.1 produces a mirror image of decimated basebandinformation signal 654.1.

The mixer 804 further down-converts the conjugated baseband image 850 tobaseband to remove residual frequency offsets such as frequency offsetof ω_(t) to provide an example. More specifically, the mixer 804comprises of n mixers 804.1 through 804.n produces a down-convertedbaseband image 852.1 through 852.n by down-converting the conjugatedbaseband image 850.1 through 850.n using, a carrier frequency ω_(x1)through ω_(xN), denoted as e^(jω) ^(x1) ^(t) through e^(jω) ^(xN) ^(t).In an exemplary embodiment, the frequency of carrier frequency ω_(x1)through ω_(xN) are substantially equivalent a corresponding frequency ofcarrier frequency ω_(x1) through W_(xN) as shown in FIG. 7.

The VID 808 comprises of n VIDs 808.1 through 808.n to resample acorresponding down-converted baseband image 852 from the sampling rateused by A/D 408 as shown in FIG. 4 to, for example, an integer number ofsamples per symbols as in accordance with the modulation scheme of thereceived multi-channel information signal 158. AGC module 810 comprisesof n AGC modules 810.1 through 810.n to attenuate a correspondingresampled baseband image 854.1 through 854.n. AGC 712 ensures theresampled baseband image 854.1 through 854.n have substantially equalamplitudes.

The Nyquist filter 812 produces the reference residual signal 356 byfiltering the amplitude corrected baseband image 856. More specifically,the Nyquist filter comprises n Nyquist filters 812.1 through 812.n tofilter a corresponding amplitude corrected baseband image 856.1 through856.n. The Nyquist 812 filters amplitude corrected baseband image 856with a filter whose shape matches the pulse shape of the informationchannel 150. In addition to limiting the amount of noise spectrum passedonto subsequent stages, the Nyquist 812 provides the multi-channelreceiver 106 with a stronger image signal to work with by correlatingthe amplitude corrected baseband image 856 with the pulse shape of thetransmitted communication signal over the symbol period.

FIG. 9A is an illustration of an adaptive image canceler according to anexemplary embodiment of the present invention. In an exemplaryembodiment, adaptive image canceler 900 may be used to implement imagecanceler 308.1 through 308.n as shown in FIG. 3A.

The method to estimate a signal corrupted by an image is to pass thecomposite signal through an image canceler that tends to suppress theimage while leaving the signal relatively unchanged. Image canceling, isa variation of optimal filtering that uses an auxiliary or referenceinput, denoted as i₁. This input is filtered by an image canceler module904 and subtracted from a primary input, denoted as s+i₀, containingboth the signal and the image by a summer 906. As a result, the image,i₀, is attenuated or eliminated by cancellation. The image canceler 904may be implemented using a suitable adaptive filter including multipleequalization taps that adjusts equalization coefficients correspondingto the equalization taps to cause an output y to be a best least-squaresfit of the image i₁.

Still referring to FIG. 9A, a receiver receives the signal s plus anuncorrelated image i₀. The combined signal and image, s+i₀, form the“primary input” to the canceler. A receiver generates an image i₁ whichis uncorrelated with the signal s but correlated with the image i₀. Theimage i₁ is filtered to produce an output y that is a close replica ofi₀. This output is subtracted from the primary input s+i₀ to produce thesystem output, s+i_(0-y). The image i₁ is processed by the imagecanceler module 904 that automatically adjusts its own impulse responsethrough a least-squares algorithm, such as the widely known Least MeanSquared (LMS) or Recursive Least Squares (RLS) algorithms, whichresponds to an error signal c dependent on the image canceler's outputy. Thus with the proper algorithm, the image canceler can operate underchanging conditions and can readjust itself continuously to minimize theerror signal.

In image canceling systems based on a least squares criterion, thepractical objective is to produce a system output, s+i_(0-y), that is abest fit in the least-squares sense to the signal s. This objective isaccomplished by feeding the system output ε back to the image cancelermodule 904 and adjusting the image canceler module 904 through anadaptive algorithm to minimize the total system output power. Adjustingor adapting the image canceler module 904 to minimize the total outputpower is tantamount to causing the output ε to be a best least-squaresestimate of the signal s for the given structure and adjustability ofthe adaptive image canceler module 904 and for the given image i₁. Theoutput ε will generally contain the signal s with some of the image i₀remaining. Minimizing the total output power minimizes the output noisepower and, since the signal in the output remains constant, minimizingthe total output power maximizes the output signal-to-noise ratio. Imagecanceling systems are further described in Bernard Widrow & Samuel D.Stearns, Adaptive Signal Processing 302-361 (1985), which isincorporated herein by reference in its entirety.

FIG. 9B is an illustration of an adaptive image canceler according to anexemplary embodiment of the present invention. In an exemplaryembodiment, an adaptive image canceler 901 may be used to implementimage canceler 308.1 through 308.n as shown in FIG. 3A. The imagecanceler 901 operates in a substantially similar manner as imagecanceler 900 except the image canceler 901 contains a feed forwardequalizer (FFE) 902 and a decision feedback equalizer (DFE) 908.

The FFE 902 produces an equalized single channel information signal 950by correcting for distortion caused by the communication channel 104present the single channel information signal 354. During transmission,a propagation medium of the communication channel 104 may introducedistortion into the transmitted multi-channel information signal 156causing the information-bearing signals within the receivedmulti-channel information signal 158 to differ from theinformation-bearing signals within the transmitted multi-channelinformation signal 156. Noise, interference, signal strength variationsknown as fading, phase shift variations, or multiple path delays knownas multi-path propagation may introduce, distortion into the transmittedmulti-channel information signal 156. The FFE 902 compensates for thisdistortion in the single channel information signal 354 to produce theequalized single channel information signal 950. The correspondingsingle channel information signal 354 is processed by the FFE 902 thatautomatically adjusts its own impulse response through, for example, aleast-squares algorithm, such as the Least Mean Squared (LMS) or theRecursive Least Squares (RLS) algorithms, that responds to the equalizedsingle channel information signal 950.

The equalized single channel information signal 950 represents thecombined signal and image, s+i₀, or the “primary input” as demonstratedin FIG. 9A. The image i₁, denoted as the reference residual signal 356in FIG. 9B, is filtered to produce an output y, denoted as imagecanceler output 952 in FIG. 9B. The image canceler output 952 issubtracted from equalized single channel information signal 950 toproduce an image corrected single channel information signal 954. Thereference residual signal 356 is processed by the image canceler module904 that automatically adjusts its own impulse response through, forexample, a least-squares algorithm, such as the widely known Least MeanSquared (LMS) or Recursive Least Squares (RLS) algorithms, that respondsto the image corrected single channel information signal 954 dependenton the image canceler output 952.

The DFE 908 produces the multi-channel information signal 152 bycorrecting for distortion caused by the communication channel 104present the image corrected single channel information signal 954.During transmission, a propagation medium of the communication channel104 may introduce distortion into the transmitted multi-channelinformation signal 156 causing the information-bearing signals withinthe received multi-channel information signal 158 to differ from theinformation-bearing signals within the transmitted multi-channelinformation signal 156. Noise, interference, signal strength variationsknown as fading, phase shift variations, or multiple path delays knownas multi-path propagation may introduce distortion into the transmittedmulti-channel information signal 156. The DFE 908 compensates for thedistortion in the image corrected single channel information signal 954to produce the multi-channel information signal 152. The correspondingimage corrected single channel information signal 954 is processed bythe DFE 908 that automatically adjusts its own impulse response through,for example, a least-squares algorithm, such as the widely known LeastMean Squared (LMS) or Recursive Least Squares (RLS) algorithms, thatresponds to the multi-channel information signal 152.

FIG. 10 illustration of an adaptive image canceler according to afurther exemplary embodiment of the present invention. An adaptive imagecanceler 1000 may be used to implement the image canceler 308.1 through308.n as shown in FIG. 3A The adaptive image canceler 1000 removes ofcancels the corresponding residual signal 356 as previously shown inFIG. 8A and FIG. 8B from the corresponding single channel informationsignal 354 to produce the multi-channel information signal 152. Theadaptive image canceler 1000 includes the FFE 902, the image cancelermodule 904, the summer 906, the DFE 908, a multiplier 1002, a switch1004, a complex multiplier 1006, a numerically controlled oscillator(NCO) 1008, a loop filter 1010, a slicer 1012, a phase detector 1014, asummer 1016, a summer 1018, a switch 1020, and a constant modulusalgorithm (CMA) error computation module 1022.

The FFE 902 produces an equalized single channel information signal 1050by correcting for distortion caused by the communication channel 104present in the single channel information signal 354. Duringtransmission, a propagation medium of the communication channel 104 mayintroduce distortion into the transmitted multi-channel informationsignal 156 causing the information-bearing signals within the receivedmulti-channel information signal 158 to differ from theinformation-bearing signals within the transmitted multi-channelinformation signal 156. Noise, interference, signal strength variationsknown as fading, phase shift variations, or multiple path delays knownas multi-path propagation may introduce distortion into the transmittedmulti-channel information signal 156. The FFE 902 compensates for thisdistortion in the single channel information signal 354 to produce theequalized single channel information signal 1050. The correspondingsingle channel information signal 354 is processed by the FFE 902 thatautomatically adjusts its own impulse response through, for example, aleast-squares algorithm, such as the widely known Least Mean Squared(LMS) or Recursive Least Squares (RLS) algorithms, that responds to animage corrected equalized single channel information signal 1052dependent on an equalized single channel information signal 1050.

The summer 906 combines the equalized single channel information signal1050 with an image canceler output 1056 to produce the image correctedequalized single channel information signal 1052. The image correctedequalized single channel information signal 1052 includes substantiallyless interference and distortion caused by, but not limited to, thefrequency-inverted duplicate or mirror image of a corresponding signalof interest 202.1 through 202.n, one or more neighboring informationchannels within the multi-channel information signal 152, or any othersuitable source to provide some examples when compared to the singlechannel information signal 354.

A fine carrier correction loop determines the signal used by the NCO1008 to ensure the image corrected baseband information channel 160 issubstantially downconverted to baseband by multi-channel receiver 106.In other words, the fine carrier correction loop finely compensates forfrequency offsets in the image corrected equalized single channelinformation signal 1052 by removing residual frequency offsets such asfrequency offset of ω_(t) present in multi-channel receiver 106. Thefine carrier correction loop of the adaptive image canceler 1000includes the complex multiplier 1006, the NCO 1008, the loop filter1010, the slicer 1012, the phase detector 1014, and the summer 1018.

The complex multiplier 1006 multiplies the image corrected equalizedsingle channel information signal 1052 with a fine carrier frequencyadjustment 1060 to produce a derotated image corrected signal 1070. Inother words, a frequency offset resultant from either the multi-channeltransmitter 102 and/or multi-channel receiver 106 may rotateconstellation points in the constellation diagram of the single channelinformation signal 354. A constellation diagram is a representation of adigital modulation scheme in the complex plane. For example, the unknownphase offset in the equalized output for a 16-quadrature amplitudemodulation (QAM) communication signal may rotate the sixteenconstellation points an amount related to the unknown phase offset. Thecomplex multiplier 1006 multiplies the symbol content of the an imagecorrected equalized single channel information signal 1052 by finecarrier frequency adjustment 1060 to rotate the constellation points inthe constellation diagram in the opposite direction as the frequencyoffset.

The summer 1018 combines the derotated image corrected signal 1070 withthe image corrected baseband information channel 160 to produce a softdecision 1066. The soft decision 1066 represents a downconvertedrepresentation of the received multi-channel information signal 158 thathas undergone the process of image cancellation by the adaptive imagecanceler 1000. The information-bearing signals within the receivedmulti-channel information signal 158 differs from theinformation-bearing signals within the transmitted multi-channelinformation signal 156 because of presence of noise in the communicationchannel 104, interference resulting from images created during themodulation and/or demodulation process to provide some examples. Theslicer 1012 uses the soft decision 1066 to produce a hard decision 1068.More specifically, the slicer 1012 estimates the data content of thesingle channel information signal 354 based upon the soft decision 1066according to a transfer function.

The phase detector 1014 generates a phase detector output 1064 that isproportional to the phase difference between the soft decision 1066 andthe hard decision 1068. When the signal generated by the phase detector1014 is approximately zero, the hard decision 1068 approximately equalsthe residual equency offsets present in multi-channel receiver 106. Theloop filter 1010 produces the loop filter output 1076 by integrating thephase detector output 1064. The loop filter output 1076 determines thefrequency and phase of the signal generated by NCO 1008. The NCO 1008generates the fine carrier frequency adjustment 1060 based upon the loopfilter output 1076 to compensate for any fine carrier offset to ensurethe single channel information signal 354 is converted to baseband atthe input to the slicer 1012. In an exemplary embodiment, the NCO 1008may be implemented using a Direct Digital Frequency Synthesizer (DDFS).In another exemplary embodiment, the fine carrier frequency adjustment1060 may also be used by the DC bias and imbalance correction module 304and/or, digital front, end 306 to remove residual frequency offsets suchas frequency offset of ω_(t) shown in FIG. 4 to provide an example.

The summer 1016 combines the soft decision 1066 with the hard decision1068 to produce a least-squares error 1074. The switch 1020 coupled tothe summer 1016 switches between the least-squares error 1074 and a CMAerror 1074 to produce an adjustment signal 1062. The FFE 902, the imagecanceler module 904 when operating in a decision directed mode, and theDFE 908 may use the adjustment signal 1062 or a re-rotated least-squareserror 1054 to automatically adjust their own impulse response accordingto a least-squares algorithm, such as the widely known Least MeanSquared (LMS) or Recursive Least Squares (RLS) algorithms. Themultiplier 1002 multiplies the re-rotated least-squares error 1054 withthe fine carrier frequency adjustment 1060 to produce the re-rotatedleast-squares error 1054. The least-squares algorithm updates adaptivecoefficients within these modules to minimize the adjustment signal 1062in the mean squared sense.

The image canceler module 904 may operate in a coarse acquisition mode,the decision directed mode, a constant modulus algorithm (CMA) mode ofoperation, and/or any combination of these modes. The switch 1004switches between the coarse acquisition mode and the decision directedmode by switching an image canceler error 1058 between the re-rotatedleast-squares error 1054 and the image corrected equalized singlechannel information signal 1052. The image canceler module 904 operatesin the coarse acquisition mode when the image corrected equalized singlechannel information signal 1052 is fed back to the image canceler module904 via the switch 1004. In the coarse acquisition mode, the imagecanceler module 904 operates in a substantially similar manner asdemonstrated in FIG. 9A. The coarse acquisition mode allows for theimage canceler module 904 to automatically adjust its own impulseresponse through, for example, a least-squares algorithm, such as thewidely known Least Mean Squared (LMS) or Recursive Least Squares (RLS)algorithms, for a larger frequency offset of ω_(t) as compared to thedecision directed mode. On the other hand, the image canceler module 904operates in the decision directed mode when the derotated re-rotatedleast-squares error 1054 is fed back to the image canceler module 904via the switch 1004. A larger signal-to-noise ratio for multi-channelreceiver 106 may be obtained by using the decision directed mode whencompared against the coarse acquisition mode. When operating accordingto the constant modulus algorithm, the image canceler module 904, theswitch couples the adjustment signal 1062 to the output of the CMA errorcomputation module 1022, denoted as the CMA error 1074 in FIG. 10. TheCMA is further described in detail in D. N. Godard, “Self-recoveringequalization and carrier tracking in two-dimensional data communicationsystems,” IEEE Transactions on Communications, vol 28, no. 11, pp.1867-1875, November 1980 and/or J. R. Treicher, B. G. Agee, “A newapproach to multipath correction of constant modulus signals,” IEEETransactions on Acoustics, Speech, and Signal Processing, vol. ASSP-31,no. 2, pp. 459-472, April, 1983, both of which are incorporated hereinby reference in their entirety.

The adaptive image canceler 1000 may be fractionally spaced tosubstantially reduce aliasing. The simplest fractionally spacedstructure is T/2 spacing, that is, 2 samples per symbol in the adaptiveimage canceler module 904. The FFE 902 does not need to be fractionallyspaced. The adaptive image canceler 1000 may also be slightlyfractionally spaced using, but not limited to, 8 T/9 spacing, 4 T/5spacing, or any other suitable spacing to provide some examples.

FIG. 11 is an illustration of an adaptive image canceler according to anadditional exemplary embodiment of the present invention. An adaptiveimage canceler 1100 may be used to implement the image canceler 308.1through 308.n as shown in FIG. 3A. The adaptive image canceler 1100removes or cancels the corresponding residual signal 356 as previouslyshown in FIG. 8A and FIG. 8B from the corresponding single channelinformation signal 354 to produce the multi-channel information signal152.

The image canceler 1100 operates in a substantially similar manner asthe image canceler 1100 except the image canceler 1100 contains a summer1102 and an image canceler module 1104. The image canceler module 1104operates in a substantially similar manner as the image canceler module904 except the image canceler module 1104 includes a first imagecanceler output 1152 and a second image canceler output 1154. The imagecanceler 1104 may use a first set of equalization coefficients togenerate the first image canceler output 1152 and a second set ofequalization coefficients to generate the second image canceler output1154. Alternatively, the image canceler 1104 may use a first set ofequalization taps to generate the first image canceler output 1152 and asecond set of equalization taps to generate the second image canceleroutput 1154.

The summer 1102 combines the single channel information signal 354 witha first output 1152 of the image canceler module 1104 to produce asingle channel information signal 1150. The single channel informationsignal 1150 includes substantially less interference and distortioncaused by, but not limited to, the frequency-inverted duplicate ormirror image of a corresponding signal of interest 202.1 through 202.n,one or more neighboring information channels within the multi-channelinformation signal 152, or any other suitable source to provide someexamples when compared to the single channel information signal 354. TheFFE 902 produces the equalized single channel information signal 1050 bycorrecting for remaining distortion caused by the communication channel104 present in the single channel information signal 1150.

The summer 906 combines the equalized single channel information signal1050 with the second image canceler output 1154 to produce the imagecorrected equalized single channel information signal 1052. In anexemplary embodiment, the summer 906 is optional. For this exemplaryembodiment, the equalized single channel information signal 1050 is usedas an input to the summer 1006 and for updating the image cancelermodule 1104 when operating in the coarse acquisition mode.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

What is claimed is:
 1. An adaptive image canceler, comprising: anadaptive filter configured to receive a received communication signal,the received communication signal differing from a transmittedcommunication signal; an image canceler module configured to produce anoutput that is a substantially close replica of a difference between thereceived communication signal and the transmitted communication signal;and a summer module configured to combine an output of the adaptivefilter with the output of the image canceler module to substantiallyremove the difference from the received communication signal.
 2. Theadaptive image canceler of claim 1, wherein the adaptive filtercomprises: at least one equalization tap configured to adaptively filterthe received communication signal in accordance with at least onecoefficient.
 3. The adaptive image canceler of claim 2, wherein the atleast one equalization tap is arranged to form a feed forward equalizer(FFE).
 4. The adaptive image canceler of claim 1, wherein the imagecanceler module comprises: a second adaptive filter, having at least oneequalization tap, configured to adaptively filter its input inaccordance with at least one coefficient.
 5. The adaptive image cancelerof claim 1, wherein the image canceler module is further configured toreceive an estimate of the difference that is correlated with thedifference and uncorrelated with a signal of interest present within thereceived communication signal as its input.
 6. The adaptive imagecanceler of claim 5, wherein the image canceler module is furtherconfigured to adaptively filter the estimate of the difference inaccordance with at least one coefficient to produce the output that isthe substantially close replica of the difference.
 7. The adaptive imagecanceler of claim 1, further comprising: a decision feedback equalizer(DFE) configured to receive an output of the summer module.
 8. Theadaptive image canceler of claim 1, wherein the difference represents animage of a signal of interest present within the received communicationsignal or distortion present within the received communication signal.9. A multi-channel receiver, comprising: a digital front end configuredto separate a received multi-channel communication signal having aplurality of information channels into a plurality of informationsignals, each of the plurality of information signals corresponding toone of the plurality of information channels; a reference generatorconfigured to provide a plurality of residual signals, each of theplurality of residual signals representing a difference between aninformation channel from among the plurality of information channels astransmitted and the information channel as received; and a plurality ofadaptive image cancelers configured to generate an output that is asubstantially close replica of a corresponding one of the plurality ofresidual signals and substantially remove the corresponding one of theplurality of residual signals from a corresponding one of the pluralityof information signals based on the output to provide a plurality ofsecond information signals.
 10. The multi-channel receiver of claim 9,wherein at least one of the plurality of adaptive image cancelerscomprises: an adaptive filter configured to receive the correspondingone of the plurality of information signals; an image canceler moduleconfigured to produce the output that is the substantially close replicaof the corresponding one of the plurality of residual signals; and asummer module configured to combine an output of the adaptive filterwith the output of the image canceler module to substantially remove thecorresponding one of the plurality of residual signals from thecorresponding one of the plurality of information signals.
 11. Themulti-channel receiver of claim 10, wherein the adaptive filtercomprises: at least one equalization tap configured to adaptively filterthe corresponding one of the plurality of information signals inaccordance with at least one coefficient.
 12. The multi-channel receiverof claim 10, wherein the image canceler module comprises: a secondadaptive filter, having at least one equalization tap, configured toadaptively filter the corresponding one of the plurality of residualsignals in accordance with at least one coefficient.
 13. Themulti-channel receiver of claim 10, wherein the at least one of theplurality of adaptive image cancelers further comprises: a decisionfeedback equalizer (DFE) configured to receive an output of the summermodule.
 14. The multi-channel receiver of claim 9, wherein the digitalfront end comprises: a plurality of multipliers configured todownconvert the received multi-channel communication signal inaccordance with a plurality of downconversion frequencies to provide aplurality of baseband information channels; a plurality of half-bandfilters configured to filter the plurality of baseband informationchannels to provide a plurality of filtered information signals, each ofthe plurality of half-band filters having a bandwidth that is greaterthan a bandwidth of a corresponding baseband information channel fromamong the plurality of baseband information channels; a plurality ofdecimators configured to reduce bandwidths of the plurality of filteredinformation signals to provide a plurality of decimated basebandinformation signals; and a plurality of Nyquist filters configured tofilter the plurality of decimated baseband information signals toprovide the plurality of information signals, each of the plurality ofNyquist filters having a shape that substantially matches a pulse shapeof the multi-channel communication signal.
 15. The multi-channelreceiver of claim 9, wherein the reference generator comprises: aplurality of automatic gain control modules configured to amplify or toattenuate magnitudes of the plurality of second information signals toprovide the plurality of residual signals.
 16. An adaptive imagecanceler, comprising: an image canceler module configured to adaptivelyfilter its input in accordance with at least one coefficient to producean output that is a substantially close replica of a difference betweena received communication signal and a transmitted communication signal;and a summer module configured to combine the received communicationsignal with the output of the image canceler module to substantiallyremove the difference from the received communication signal, whereinthe image canceler module is configured to update the at least onecoefficient based upon an output of the summer module in a coarseacquisition mode or based upon an estimation of data content of thereceived communication signal in a fine acquisition mode.
 17. Theadaptive image canceler of claim 16, wherein the image canceler modulecomprises: an adaptive filter, having at least one equalization tap,configured to adaptively filter its input in accordance with the atleast one coefficient.
 18. The adaptive image canceler of claim 16,wherein the image canceler module is further configured to receive anestimate of the difference that is correlated with the difference anduncorrelated with a signal of interest present within the receivedcommunication signal as its input.
 19. The adaptive image canceler ofclaim 17, wherein the image canceler module is further configured toadaptively filter the estimate of the difference in accordance with theat least one coefficient to produce the output that is the substantiallyclose replica of the difference.
 20. The adaptive image canceler ofclaim 16, further comprising: a slicer configured to estimate the datacontent of the received communication signal in the fine acquisitionmode.